This invention relates to belted pneumatic tires of the class generally identified as "steel belted" tires, and to novel tire cord fabrics suited for use as the reinforcement means in each belt ply of such tires.
In a belted pneumatic tire (depending on the construction of the tire, the belt structure thereof may include one, two or more individual plies in a variety of superimposed arrangements none of which constitutes any part of the present invention), all the cords in each belt ply are generally made of the same material, i.e. either metal wire such as steel or non-metallic synthetic fiber such as rayon, glass fiber, "Kevlar" (the term "Kevlar" is a trademark of E. I. du Pont de Nemours & Co. and refers to the aramid (aromatic polyamide) fiber recently developed by du Pont and initially called fiber B.) and the like. The cords in each ply are oriented substantially parallel to one another and at an angle between about 0.degree. and about 35.degree. to the median equatorial or mid-circumferential plane of the tire. Tires with belts of one type of cord material will, of course, have different properties than tires with belts of another type of cord material.
Normally the belt plies of tire, and their respective reinforcing cords, are under tension in use and accordingly the cords experience large tension stresses along their lengths. However, during cornering, the belt plies and their respective cords are subject to large compression stresses as well. One of the virtues of steel belts is that by the inherent stiffness of steel, they resist buckling and bird-caging (i.e. the cords spreading open in all directions leaving a void without cords) in cornering due to such compression. Textile filaments because of their low inherent stiffness and high twist are not able to so resist compression forces. Thus in cornering they compress, buckle and birdcage. Such buckling and birdcaging cause excessive tire tread wear and tire failure.
This is particularly evident when the belt cords are formed solely of glass fibers. Glass cords when subjected to sub-zero weather conditions, such as found in the northern United States and Canada, becomes brittle and subject to high-rates of breakage in normal highway use. While bias ply tires having belts composed solely of glass have been in existence for approximately 10 years, the problem of cord breakage has only recently come to light. In the case of bias belted tires the problem of breakage due to compression is minimal since the belt angles are only slightly lower than the carcass angles. Thus in such bias ply tire, breakage of the belt cords due to compression results only in weight non-uniformity, groove cracking and ultimately tire tread separation. In radial tires, the problem caused by the breakage due to compression becomes greatly intensified since the belt angles are much different than the carcass angles. Thus, when breakage of the individual belt cords occur, they lead to blowouts and other forms of extreme tire failure.
This only has become clearly evident since the recent introduction of radial tires having belts reinforced only with pure glass cords. High failure rates, particularly in cold climates, were found upon the introduction of such radial tires.
Testing of such radial tires indicate their lack of reliability. For example, radial tires of this type were tested by placing them in a cooling unit until they reached minus 30.degree. F. They were then mounted on automobiles and allowed to warm to minus 25.degree.. They were thereupon driven five miles in a city environment and across at least one railroad track. Each such cooling and driving period was considered to constitute on cycle. After only one cycle most such tires had large numbers of broken cords. Many tires failed in just one such cycle. Better quality tires having belts with only glass cords lasted from two to five cycles before serious damage or failure.
Such breakage, with the concurrent possibility of extreme tire failure and blowouts, make radial tires with belts having all glass cords all but totally unacceptable.
In belt cords composed of other high modulus non-metallic cords, buckling and birdcaging, while not leading to such extreme failures, do lead to a decrease or tread life. For example when the belt cords are made of Kevlar, tread life average is 7% below that of tires with only steel belt cords.